GENETIC STUDIES OF RABBITS AND RATS. 6 



(which is supposed to be complete at one year) or for bone- 

 dimensions, which as a rule are based on specimens fourteen months 

 old. 



GROWTH-CURVES BASED ON WEIGHT. 



Most of the rabbits were born in the spring and summer months, 

 May to August inclusive, but some were born as late in the year as 

 November and a few in the early part of December. None was born 

 in the months January to March inclusive. The young were usually 

 weaned when one month old, at which time periodical weighing of 

 the animals was begun. It was considered very desirable to raise all 

 the individuals born, for fear that if small-sized individuals should 

 succumb in competition with their larger brothers and sisters, the 

 statistical conclusions might be vitiated thereby. To this end, free 

 use was made of foster-mothers. Young under a week old may 

 readily be transferred from the nest of one mother to that of another. 

 In only one case have I known a mother to refuse to care for foster- 

 children substituted for her own of like age. Of course, notwith- 

 standing all our precautions, many young rabbits died before reaching 

 maturity, either from intestinal troubles in hot weather or from 

 nasal troubles ("snuffles") in cold weather. But a careful study of 

 our records indicates that there was no differential selection in favor 

 either of large or of small rabbits in these deaths. The young rabbits 

 were supplied with an abundance of suitable food, in summer grass 

 and oats, in winter hay, oats, and cabbage or carrots. Occasionally 

 a rabbit was not weaned promptly at one month of age. If, for 

 example, an individual seemed feebler than its fellows at weaning- 

 time, it might be left with the mothfer a week or two longer. Again, 

 differences in size were often observed at weaning-time between 

 litters of like ancestry, but of unlike number in individuals. If a 

 doe nurses six young, they will average smaller at weaning than will 

 young of similar ancestry nursed by two mothers, each of which 

 divides her milk among three young. Fanciers believe that this 

 affects the ultimate size of the offspring. They think that the 

 largest rabbits are obtained by rearing only two or three young to a 

 litter and by allowing these to nurse the mother for six weeks or 

 two months. Our observations do not support this view, but indicate 

 that the ultimate size attained is influenced very little, if at all, 

 by the size of the young at weaning, provided they are in good health 

 and growing condition and are thereafter fed abundantly. 



Figure 1 illustrates the matter well. Rabbits 2650 and 2834 were 

 half-brothers. They had the same pure Flemish father and their 

 mothers were of the same pure Polish stock. Rabbit 2650 was 

 weaned at 29 days of age and grew slowly until he was 71 days old, 

 when he weighed 600 grams. Rabbit 2834 was raised by a foster- 



